1. Field of the Invention
The present invention relates to an acceleration sensor which may suitably be used in a webbing retractor installed in a vehicle and adapted to detect an emergency experienced by the vehicle from the acceleration acting in the horizontal direction, and suspend the reeling out of the webbing upon detection.
2. Description of the Related Art
A webbing retractor of the above-described type has an acceleration sensor provided for sensing the acceleration of the vehicle that acts in the horizontal direction. The acceleration sensor has a mass body which is movable on an inclined surface, and an arrangement of the sensor is known in which, when the mass body moves in the horizontal direction to climb the inclined surface, a lever-shaped pawl is actuated by the action of the mass body to suspend the rotation of a lock ring, thereby instantly suspending the reeling out of the webbing from the webbing retractor.
With this arrangement of the acceleration sensor, however, the pawl is also actuated when the mass body moves vertically. With the above-described acceleration sensor, therefore, when an acceleration acts in the vertical direction, the reeling out of the webbing is suspended even though the vehicle is not experiencing any emergency.
Such a phenomenon often takes place when the vehicle is running on a road having a rough surface or if the vehicle is a vehicle such as a motor-truck having a large suspension spring constant. More specifically, the phenomenon occurs when, due to a factor such as vibration of the vehicle, the mass body jumps upward from the inclined surface or undergoes rolling movement on the inclined surface. In such a case, the vertical acceleration is much greater than a horizontal acceleration of an ordinary magnitude.